Nature of optical transitions involving cation vacancies and complexes in AlN and AlGaN

Abstract

Photoluminescence spectroscopy was employed to probe the nature of optical transitions involving Al vacancy (VAl) and vacancy-oxygen complex (VAl-ON) in AlN. An emission line near 2 eV due to the recombination between the 2− charge state of (VAl-ON)2−/1−, and the valence band was directly observed under a below bandgap excitation scheme. This photoluminescence (PL) band was further resolved into two emission lines at 1.9 and 2.1 eV, due to the anisotropic binding energies of VAl-ON complex caused by two different bonding configurations–the substitutional ON sits along c-axis or sits on one of the three equivalent tetrahedral positions. Moreover, under an above bandgap excitation scheme, a donor-acceptor pair like transition involving shallow donors and (VAl-ON)2−/1− deep acceptors, which is the “yellow-luminescence” band counterpart in AlN, was also seen to split into two emission lines at 3.884 and 4.026 eV for the same physical reason. Together with previous results, a more complete picture for the optical transitions involving cation vacancy related deep centers in AlGaN alloy system has been constructed.